This invention relates to a magnetoresistive sensor for measuring relative displacements of construction parts, comprising a magnetoresistive element formed on a surface of a substrate, and a magnet system which generates a magnetic field having a component directed parallel to the substrate surface at the area of the magnetoresistive element. The field component, upon the relative displacement of the construction parts, extends at different angles xcex1 to the magnetoresistive element and is so large that the magnetoresistive element is always magnetically saturated during the displacement.
A magnetoresistive sensor of this type may be used for determining the mutual position of two construction parts in a contactless way, with the magnetoresistive element being secured to one of the construction parts and the magnet system being secured to the other part. The construction parts may be, for example, a housing and a cam shaft rotatable therein, as is customary in motorcar engines. The construction parts may also be, for example, a rail system and a slide which is movable across this system, as is customary in movable car seats. In the first-mentioned case, the displacement is a rotation, in the last-mentioned case, it is a translation.
During the displacement of the construction parts, the magnetoresistive element is always magnetically saturated by the magnetic field component directed parallel to the surface of the substrate of the magnetoresistive element. The magnetoresistive element then has a resistance which only depends on the angle xcex1 at which the element extends to the component of the magnetic field. Moreover; variations of the magnetic field strength which may be caused, for example, by temperature fluctuations substantially do not influence the operation of the sensor and mechanical tolerances may be relatively large, both in the manufacture and during use of the sensor.
A magnetoresistive sensor of the type described in the opening paragraph is known from Philips Semiconductors Data Handbook SC 17, Product Specification, Dec. 6, 1997, pp. 183-187, particularly FIGS. 2 and 3, which sensor is used for determining the rotation of a shaft. The magnet system is a permanent magnet which is secured to the end of the shaft, and the magnetoresistive element is arranged at some distance from the end of the shaft. The magnetic field extends at an angle xcex1 to the magnetoresistive element. The centerline of the shaft perpendicularly intersects the surface on which the magnetoresistive element is formed. The magnetoresistive element has a resistance which is dependent on sin2xcex1. Therefore, only rotations up to 90xc2x0 can be unambiguously measured with the sensor.
It is, inter alia, an object of the invention to provide a magnetoresistive sensor allowing measurements of displacements in which the extent of the displacement is less limited than in the known magnetoresistive sensor.
According to the invention, the magnetoresistive sensor is therefore characterized in that, upon relative displacement of the construction parts, the magnetoresistive element describes a path with respect to the magnet system.
In the known sensor described above, the centerline of the shaft on which the magnet is secured perpendicularly intersects the surface on which the magnetoresistive element is formed. Upon rotation of the shaft, the magnetic field rotates around this point of intersection. With respect to the magnet system, the magnetoresistive element rotates about the same point of intersection and does not change its position. In the sensor according to the invention, the magnetoresistive element does not maintain its position with respect to the magnet system but describes a path with respect to the magnet system. In practice, the magnetoresistive element will generally be stationary, whereas the magnet system changes position, but for a better understanding of the invention it will be clearer if the relative displacement is described as a displacement of the magnetoresistive element with respect to the magnet system. Since the magnetoresistive element describes a path with respect to the magnet system, the magnet system may be designed in such a way that a desired magnetic field is generated at each point of the path. Large translations and large rotations up to substantially 360xc2x0 can be unambiguously measured with such a sensor.
Upon the relative displacement of the construction parts, said component of the magnetic field preferably extends at angles xcex1 to the magnetoresistive element, which angles are within a work range in which the magnetoresistive element has a resistance depending substantially linearly on these angles. The magnet system can then be designed in such a way that large translations and large rotations up to substantially 360xc2x0 can be measured in a linear manner.
In a magnetoresistive sensor for determining a mutual rotation of the construction parts, the magnetoresistive element preferably describes a circular path with respect to the magnet system, with a center with respect to which the magnetoresistive element occupies a fixed position. One of the construction parts is arranged, for example, fixedly, whereas the other construction part can rotate with respect to the fixed construction part. Either the magnetoresistive element or the magnet system is then secured to the rotating construction part. When the magnetoresistive element or the magnet system is directly secured to the rotating construction part, for example, by means of a fixed arm, then it automatically describes a circular path with respect to the fixed construction part. Gears with, for example, cogs are not necessary for realizing the circular path. Rotations up to substantially 360xc2x0 can be measured.
In a first embodiment of the magnetoresistive sensor, the magnet system generates a magnetic field, with field lines which radially extend in the plane of the path with respect to an axis of symmetry directed transversely to the plane of the path and intersecting the plane of the path next to the center of the path. The magnetic field component directed parallel to the surface of the substrate extends along the circular path at different angles ox to the magnetoresistive element. The distance between the center of the path and the point of intersection of the symmetry axis with the plane of the path determines the work range within which the angle xcex1 varies during the rotation of the construction parts. This distance may simply be chosen to be such that this angle xcex1 remains in said work range during the rotation.
If the construction parts rotate at an angle xcex2 with respect to each other, then the resistance of the magnetoresistive element changes linearly with the angle xcex1 but not with the angle xcex2 in such a magnetic field. The angle xcex1 is not linearly dependent on the angle xcex2. For the described magnetic field, in which the magnetoresistive element describes a circular path, the resistance does not appear to extend directly proportional to the angle xcex2 but substantially to the sine of the angle xcex2. A rotation of 180xc2x0 can thus be unambiguously measured with the sensor.
A magnet system with which such a field is realized in a simple manner comprises two concentrically arranged ring magnets with their polar faces located in a plane extending parallel to the plane of the path, and with a common centerline constituting the axis of symmetry of the magnet system, both ring magnets being magnetized in opposite sense transversely to the plane of the path. Such a magnet system with two ring magnets may be mounted around a shaft, with the centerline of the shaft then passing through the center of the path of the magnetoresistive element.
To ensure that, in this first embodiment, the magnetoresistive element is always saturated during the mutual rotation of the construction parts, a magnetic field which is as small as possible is sufficient if the surface of the substrate on which the magnetoresistive element is formed is located in the plane of the path of the magnetoresistive element.
In a second embodiment of the magnetoresistive sensor, the magnet system generates a magnetic field with substantially parallel field lines which intersect the plane of the path at an angle at the location of the circular path. The magnetic field component directed parallel to the substrate then extends along the circular path at different angles to the magnetoresistive element. The size of the angle at which the field lines extend to the plane of the path determines the work range within which the angle xcex1 varies during the rotation of the construction parts. This angle can be chosen to be such that the angle xcex1 remains in said work range during the rotation.
Also in this case, it holds that the resistance of the magnetoresistive element substantially changes with the sine of the angle xcex2 if the construction parts rotate at an angle xcex2 with respect to each other, so that a rotation of 180xc2x0 can be unambiguously measured also with this sensor.
A magnet system in which such a field is realized in a simple manner comprises two ring magnets which, with their polar faces facing each other, are arranged on both sides and at an equal distance from the plane of the path, with centerlines intersecting the plane of the path at points located at equal distances on both sides of the center of the path, both ring magnets being magnetized in equal sense transversely to the plane of the path. Also this magnet system, again with two ring magnets, may be mounted around a shaft, in which the centerline of the shaft then passes through the center of the path of the magnetoresistive element.
To ensure that, in this second embodiment, the magnetoresistive element is always magnetized during the mutual rotation of the construction parts, a magnetic field which is as small as possible is sufficient if the substrate surface on which the magnetoresistive element is formed extends transversely to the path of the magnetoresistive element.
Rotations up to 180xc2x0 can be measured in an unambiguous manner with the sensors of the first and the second embodiment. The field of application is extended if the sensor comprises two magnetoresistive elements which describe the same circular path with respect to the magnet system, occupy the same fixed position with respect to the center of the path and, viewed from the center, enclose an angle of 90xc2x0. The resistance of one magnetoresistive element then changes upon a mutual rotation of the construction parts by sin xcex2, while the resistance of the second magnetoresistive element, which is rotated 90xc2x0 with respect to the first element, changes by cos xcex2. The angle xcex2 can then be calculated from these two resistances over an interval of 360xc2x0. By adding a suitable electronic circuit, the rotation through 360xc2x0 can even be measured linearly.
In a magnetoresistive sensor for determining a mutual translation of the construction parts, the magnetoresistive element preferably describes a rectilinear path with respect to the magnet system, which path is located in a reference plane with respect to which the magnetoresistive element occupies a fixed position. Either the magnetoresistive element or the magnet system is then secured to the moving construction part and then moves along a rectilinear path. Gears with, for example, cogs are then not necessary for realizing the desired rectilinear path. The translation may be large.
In a third embodiment of the magnetoresistive sensor, the magnet system generates a magnetic field with field lines intersecting the reference plane at different angles at the location of the rectilinear path. It is thereby realized that the component of the magnetic field directed to the surface of the substrate extends along the path at different angles xcex1 to the magnetoresistive element. The angles at which the field lines intersect the reference plane may be simply chosen to be such that the angle xcex1 remains in said work range during the translation.
A magnet system with which such a magnetic field can easily be realized comprises two elongated magnets which, with their polar faces facing each other, are arranged on both sides of the reference plane and, viewed in projection on the reference plane, intersect each other in the center of the path and enclose an equally large, but opposite angle to the rectilinear path, both magnets being magnetized in the same direction transverse to the reference plane. The two opposite angles may be chosen to be such that the angle xcex1 remains in the desired work range.
To ensure that, in this third embodiment, the magnetoresistive element is always magnetized during the mutual translation of the construction parts, a magnetic field which is as small as possible is sufficient if the substrate surface on which the magnetoresistive element is formed extends transversely to the path of the magnetoresistive element.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.